1. Field of the Invention
The present invention relates to a carrier formed of a coated particulate core material and a method of preparing the carrier, and to a developer, a container containing the developer, an image forming method and a process cartridge.
2. Description of the Related Art
Electrophotographic image forming methods include forming an electrostatic latent image on an image bearer such as a photoconductive material, transferring a charged toner thereto to form a visible image (toner image), transferring the toner image onto a recording medium such as paper, and fixing the toner image thereon to form a final output image. Recently, electrophotographic copiers and printers are rapidly developing from monochrome to full-color, and full-color markets are expanding.
The electrophotographic image forming methods typically include overlaying three primary color toners, i.e., yellow, magenta, and cyan toners, or four color toners including the previous three and black toner, to reproduce all colors. Therefore, to produce a sharp full-color image having good color reproducibility, the surface of a fixed toner image has to be smooth to reduce light scattering. This is why many conventional full-color copiers produce images having medium to high glossiness of from 10 to 50%.
As a method of fixing a dry toner image on a recording medium, contact heat fixing methods using a heated roller or a heated belt having a smooth surface are typically used. Although the methods have high heat efficiency and are capable of fixing at high speed and imparting gloss and transparency to color toners, offset problems do occur, wherein a part of a toner image adheres to a fixing member and transfers to another image occur during separation of the fixing member from the melted toner image after pressing the surface of the fixing member.
For the purpose of preventing offset problems, the surface of the fixing member has typically been coated with silicone rubber or a fluorine-containing resin. Further, a releasing agent in the form of an oil such as silicone oil is applied to the surface of the fixing member. Although this approach is quite effectively used to prevent offset problems, a release oil applicator is required and the resultant fixer becomes larger.
Therefore, as an alternative, there are oilless arrangements using a monochrome toner having high viscoelasticity when melted so as not to breakdown internally, wherein the molecular weight distribution of a binder resin of the toner is controlled, and further including a release agent such as wax, or systems that apply only a small amount of oil.
In addition, even full-color image forming apparatuses are becoming oilless as well for the purpose of being downsized and simplified. However, as mentioned above, to improve color reproducibility of a color toner, the color toner needs to have lower viscoelasticity because the fixed color toner image is required to have a smooth surface. Therefore, the color toner has offset problems more often than the monochrome toner does, making it more difficult to make a fixer oilless or use only a small amount of oil. In addition, a toner including a release agent has higher adherence to an image bearer and lower transferability to a transfer paper. Further, the release agent therein contaminates friction-charged members such as a carrier and lowers the chargeability thereof, resulting in deterioration of durability of the toner.
On the other hand, for the purpose of preventing toner constituents from filming, making the surface thereof uniform, preventing oxidization thereof, preventing deterioration of moisture sensitivity thereof, extending lives of developers, preventing adherence of the carriers to the surfaces of photoreceptors, protecting photoreceptors from being damaged or abraded by the carriers, controlling charge polarity thereof and controlling charge quantity thereof, a resin having a low surface energy such as fluorine-containing resins and silicone resins is applied on the carrier core material to prolong the life of the carrier.
As examples of carriers coated with a resin having a low surface energy, Japanese published unexamined application No. 55-127569 discloses a carrier coated with an ordinary-temperature curing silicone resin and a positively-chargeable nitrogen resin, Japanese published unexamined application No. 55-157751 discloses a carrier coated with a coated material including at least one modified silicone resin, Japanese published unexamined application No. 56-140358 discloses a carrier having a coated layer including an ordinary-temperature curing silicone resin and a styrene acrylic resin, Japanese published unexamined application No. 57-96355 discloses a carrier coated with two or more silicone resin layers having no adherence to each other, Japanese published unexamined application No. 57-96356 discloses a carrier coated with silicone resin multiple layers, Japanese published unexamined application No. 58-207054 discloses a carrier coated with a silicone resin including a silicon carbonate, Japanese published unexamined application No. 61-110161 discloses a positively-chargeable carrier coated with a material having a critical surface tension not greater than 20 dyn/cm, and Japanese published unexamined application No. 62-273576 discloses a developer formed of a carrier coated with a material including fluorine-containing alkylacrylate and a toner including a chrome azo dye.
It is known that a coating liquid including a siloxane material having a condensation reactable silanol group or its precursors (hydrolysis groups such as halo(silyl) groups and alkoxy silyl groups) is condensed to a polysiloxane material with titanium catalysts to coat the surface of a carrier particulate core material. Japanese published unexamined application No. 2001-92189 discloses coating a particulate core material with a silicone resin including organic titanium catalysts, and discloses diisopropoxybis(acetylacetonate) disclosed in Comparative Example 2, tetraisopropoxytitanium disclosed in Comparative Example 1, isopropoxy(2-ethylhexanediolato)titanium, bis(acryloyloxy)isopropoxyisostearoyloxytitanium, bis(2,4-pentadionate) and (1,3-propanedionate)titanium as titanium catalysts. Japanese published unexamined application No. 06-222621 discloses coating the surface of a particulate core material with a composition formed of at least one curing catalyst selected from a group consisting of organopolysiloxane, organosilane, titanium such as tetraisopropoxytitanium, tin such as dibutyltindiacetate, zinc, cobalt, iron, aluminum compounds and amines. Further, Japanese published unexamined application No. 2006-337828 discloses a particulate core material that is coated with a silicone resin or a modified silicone resin including a quaternary ammonium salt catalyst, aluminum catalyst or a titanium catalyst such as disopropoxybis(acetylacetonate).
However, particulate toner has a smaller particle diameter to produce higher-quality images. Further, printing speed is constantly increasing, and therefore a phenomenon known as toner spent on carrier more easily occurs. A developer including a wax for easy maintenance sharply increases toner spent, resulting in lowering the toner charge, toner scattering, and background fouling.
In full-color electrophotographic systems, when toner spent on carrier occurs, coated layer thereof is damaged or peeled off, the carrier resistance and the developer consumption varies, and the image density, particularly the image density of a highlighted part, varies. Further, escaped filler due to the damaged coated layer contaminates the color of a toner (particularly a yellow toner), and high quality images are not reliably produced.
In addition, blocking occurs when forming a coated layer, and consequently the durability thereof deteriorates.
For these reasons, a need exists for a carrier having good toner spent resistance and abrasion (damage or peeling) resistance of its coated layer.